Heretofore, an electrophoretic device as shown in FIG. 10(a) has been known.
This electrophoretic device comprises electrodes 3 formed on a first substrate 1, transparent electrodes 4 formed on a second substrate 2, an electrophoretic dispersion 10 filled between the electrodes 3 and the transparent electrodes 4, and spacers 7 preventing the electrophoretic dispersion 10 from flowing out. In the figure mentioned above, a cross-section of one pixel of the display device is shown.
The electrophoretic dispersion 10 is formed of a liquid-phase dispersion medium 6 and electrophoretic particles 5 dispersed in this liquid-phase dispersion medium 6. In addition, the liquid-phase dispersion medium 6 and the electrophoretic particles 5 are colored so as to have colors different from each other.
To this electrophoretic device, voltage sources 9a and 9b for applying voltages, which are electrically opposite to each other, are connected via a switching element 8. That is, the electrode 3 is connected to ends of the voltage sources 9a and 9b, and the electrode 4 is connected to the other ends of the voltage sources 9a and 9b via the switching element 8. When an electrical circuit is formed as described above, the direction of a voltage to be applied can be changed using the switching element 8. By changing the direction of a voltage to be applied, the electrophoretic particles are gathered at a desired electrode side, so that a desired display can be created. That is, when the electrophoretic particles 5 are negatively charged, as shown in FIG. 10(b), by applying a voltage supplied from the voltage source 9a, the electrophoretic particles 5 can be gathered at the transparent electrode 4 side, which is close to an observer side. In this state, the observer sees the color of the electrophoretic particles 5. On the other hand, as shown in FIG. 10(c), by applying a voltage supplied from the voltage source 9b, the electrophoretic particles 5 can be gathered at the electrode 5 side, which is far from the observer side. In this state, the observer sees the color of the liquid-phase dispersion medium 6. When the electrophoretic particles 5 are positively charged, the moving direction of the particles is opposite to that described above.
As described above, when the structure shown in FIG. 10(a) is used, two colors can be displayed in accordance with the direction of a voltage to be applied, and when the structure shown in this figure is used in each pixel, an electrophoretic device can be realized.
[Problems to be Solved by the Invention]
When the electrophoretic device described above is formed on a substrate having flexibility, it is anticipated that electronic paper may be realized. However, in this case, after display information is saved by applying a voltage, it is necessary to sustain the display information for a long period of time.
A technique for sustaining the display information has been disclosed in Japanese Examined Patent Application Publication No. 50-15115. In this publication, a dispersion medium, which is solid at room temperature and is softened by heating or using solvent, is used. However, when the technique described in this publication is used, a material for the dispersion medium is limited, and in addition, a complicated step such as heating is required when images are saved.
In addition, another technique has been disclosed in Japanese Unexamined Utility Model Publication No. 2-51325. In the technique described in this publication, liquid crystal is used as the dispersion medium. In the technique described above, there has been a problem in that a material for the dispersion medium is limited.
Furthermore, in U.S. Pat. No. 5,961,804, the structure in which a charge having a polarity opposite to that of particles is kept to the inside walls of microcapsules containing a dispersion has been disclosed. However, in this case, a method for keeping a charge is difficult, and a problem may arise in that the manufacturing process becomes complicated. In addition, a problem may also arise in that a material for the microcapsules is limited.
In addition, in Japanese Unexamined Patent Application Publication No. 3-213827, a technique in which an image is refreshed by periodically applying a driving voltage has been disclosed. However, in this case, time control for periodically applying a driving voltage is complicated, and as a result, there has been a problem in that the size of an electrical circuit portion is increased. Accordingly, it has been difficult to apply these above techniques to the formation of electronic paper.
The present invention was made in order to solve the problems described above, and an object of the present invention is to provide an electrophoretic device that can be formed of a simple electrical circuit by using materials which are not so limited and that has significantly improved image-sustaining properties for realizing electronic paper.